Most existing refractive laser delivery systems provide little feedback for the operator (typically a surgeon). Usually a crosshair graticule is superimposed through the microscope optics to help the operator aim the laser beam correctly onto the cornea. A fixation target or light, such as a flashing LED, is used to ensure that the patient's eye remains correctly aligned during the surgery. However, this arrangement does not necessarily provide the best alignment of the eye and the laser beam, nor does it provide visual feedback for the operator concerning the status of the eye or the laser. It may at times be necessary for the operator to move his or her attention away from the surgical field to check on instrumentation, such as the microkeratome or the laser source. The axis of astigmatism of the patient's eye is also likely to be misaligned when the patent is supine and fixating on a point of light.
Refractive errors are usually assessed when the patient is seated in an upright position using structured shapes or symbols, such as letters of the alphabet. However, refractive surgery is usually performed with the patient reclining in an operating chair. It has been found that, when a patient lies recumbent, the ocular globe is liable to rotate, altering the position of the axis of astigmatism between 7° and 16° in 25% of cases (Smith, Talamo, Assil & Petashnick, “Comparison of Astigmatic Axis in the Seated and Supine Positions”, J. of Refractive & Corneal Surgery 10(6), 615 (1994)). This occurs for two reasons: i) the removal of the reference horizon, and ii) the change from binocular to monocular vision. Focussing on a point of light (the flashing LED), instead of the linear horizon, does not provide a proper point of horizontal or vertical reference. The globe is therefore liable to rotate fractionally, possibly resulting in misalignment of the treatment of the eye's axis of astigmatism. The potential end result is under-treatment of the original astigmatic error or inducement of astigmatism at another axis.
U.S. Pat. No. 5,549,597 describes a method for determining the axis of astigmatism of a patient undergoing refractive surgery, so as to provide real-time alignment information for the surgical procedure. The patient is required to focus on a target such as three sets of three lines of variable line spacing, each set corresponding to a different visual acuity, and then to focus on the best resolved set of lines and rotate the target until the finest line is seen most clearly. This method of determining the axis of astigmatism and aligning the surgical laser is not ideal. The patient is forced to make subjective comparisons at a highly stressful time. In addition, the globe may still rotate after the alignment has been performed, and prior to surgery.
An earlier configuration for determining the axis of astigmatism is described in U.S. Pat. No. 3,785,723, and involves rotation of an opaque disk having multiple small apertures backlit by a light source so as to resemble a set of point light sources arranged in a straight line along the diameter of the disk.
U.S. Pat. No. 5,442,412 discloses a patient responsive eye fixation target for use in ophthalmic procedures in which respective light sources produce a ring of light and a dot of light centred on the same optical axis, but respectively closer to and further from the eye. In response to detection of a quantifiable amount of eye movement, the dot is altered in appearance, eg. by flashing or colour changes, to alert the patient that his or her eye is no longer aligned with the dot and ring.
Corresponding to the patient fixation apparatus is the apparatus used by the surgeon to view and assess the extent of fixation and the alignment of the laser beam. The surgeon views this display when looking down the surgical microscope. Current technology provides a display including a graticule or crosshair. A He-Ne beam is sometimes provided for aiming the surgical beam.
U.S. Pat. No. 4,870,964 provides a head-up display for use with an operating microscope during phaco-emulsification procedures. This apparatus allows the operating surgeon to view information about the status of the patient, the eye and operating equipment, such as vacuum pressure, without removing their gaze from the operating field. It does so by projecting light onto the operating field of the eye and conditioning the reflections from the cornea so that interpretable images may be viewed by the surgeon as they look down the microscope. U.S. Pat. No. 5,135,299 describes a similar operating microscope featuring a head-up display, produced by reflecting operational information from the scleral portion of the eye.
It is an object of the present invention, in at least one aspect, to provide an eye fixation method and apparatus that is simple and reliable, and involves minimal expectation of the patient. For particular applications, it is further preferred that the arrangement reduces the angular rotation of the ocular globe to facilitate alignment of an instrument with the axis of astigmatism.
It is an object of another aspect of the present invention to provide a surgical visual feedback method and apparatus that provides increased information to the surgeon or operator.